Most microwave radar systems for low cost applications such as automotive collision warning systems use Doppler signal processing. The Doppler effect (or Doppler shift) is the change in frequency of a reflected wave (or other periodic event) for an observer moving relative to its target. Doppler signal processing may be used to separate reflecting signal from spurious noise and to measure moving target velocity. However, Doppler detection is challenging, because the return signal is from reflection not from a baseband station. Thus, a low noise Doppler detector is needed. The BARrier Injection Transit-Time (BARITT) diode, as a discrete device, has demonstrated high sensitivity in self-mixing microwave Doppler systems. However, discrete device systems have undesirably high fabrication cost and complexity for low cost applications.